Surfactants that are capable of developing viscoelasticity in aqueous solutions are of interest for a variety of wellbore fluids, such as fracturing fluids (see e.g. U.S. Pat. Nos. 5,551,516 and 6,232,274), selective fluids for water control (WO 99/50530 and U.S. Pat. No. 6,194,356), drilling fluids (Navarrete, R. C. and Wei, Z., Characteristics of viscoelastic surfactant systems in drill-in and completion fluids, Chemistry in the Oil Industry VIII Symposium, Manchester, November 2003, pp. 261-273) and the selective placement of treatment fluids, such as a scale dissolver (WO 02/12673 and U.S. Pat. No. 7,156,177).
A wide range of viscoelastic surfactant solutions have been developed and formulated, with ionic headgroups including quaternary amines (U.S. Pat. No. 5,964,295), amide and ester carboxylates (GB-A-2372058 and WO 02/064946) and amidesulphonates (GB-A-2408506). Dimer and oligomer surfactants have also been described (GB-A-2371316) and have been shown to produce minimal emulsions when their aqueous solutions are mixed with hydrocarbons. Viscoelastic surfactants have also been produced in a dry powder form as a convenient form of delivery (GB-A-2372518).
The reaction between ethanolamines and fatty acids was first reported in Koganei, R., On fatty acids obtained from cephalin. Compounds of β-aminoethyl alcohol with saturated and unsaturated fatty acids, J. Biochem., 3, 15-26 (1923). Since then, the surfactants formed by the neutralisation reaction between fatty acids, particularly stearic, oleic and palmitic acids, and ethanolamine bases, have been used in a variety of personal care products. Typical examples are aerosol shaving foams (U.S. Pat. No. 2,655,480), creams (GB-A-2282386), soaps (Trusler, R. B., Ethanolamine soaps, Ind. Eng. Chem., 21, 685-687 (1929)) and skin cleansers (GB-A-2343189). The reaction of triethanolamine with the fatty acids found in human skin sebum to form a soap for cleansing and acne prevention has recently been evaluated (Musial, W. and Kubis, A., Preliminary assessment of alginic acid as a factor buffering triethanolamine interacting with artificial skin sebum, Eur. J. Pharmaceutics Biophamaceutics, 55, 237-240 (2003)).
Ethanolamine stearates have been used in a variety of industrial applications. U.S. Pat. No. 2,353,830 describes an air pump lubricant that consists of an oil-in-water emulsion stabilised with triethanolamine stearate and free stearic acid. U.S. Pat. No. 2,581,132 describes a lubricating oil consisting of a water-in-oil emulsion, a substituted glyoxalidine and the alkylolamine salt of a fatty acid, such as stearic acid. The fatty acid salts of ethanolamines have been described as wax crystal modifiers in fuel oils (GB 1318241) and as emulsifiers to enable the application of molten wax dispersed in water to coat underwater surfaces to inhibit corrosion and fouling (U.S. Pat. No. 4,183,757).
Desirably, viscoelasticity should be maintained over a temperature range, e.g. extending from ambient up to 150° C., to match the conditions that wellbore fluids can experience downhole. However, the maximum viscosity of conventional wellbore fluid surfactant solutions is generally attained at ambient temperature and increasing temperatures are usually accompanied by a systematic decrease in viscosity.
The oil industry has used ethanolamines for many years as corrosion inhibitors and scavengers for hydrogen sulphide and carbon dioxide (Gregory, L. B. and Scharmann, W. G., Carbon dioxide scrubbing by amine solutions, Ind. Eng. Chem., 29, 514-519 (1937)). The reaction between ethanolamines and acid gases, such as hydrogen sulphide, is well known and they are used in their basic form rather than reacted with fatty acids. The ethanolamine salts of fatty acids have found several applications in wellbore fluids over a number of years. U.S. Pat. No. 2,265,962 describes the use of fatty acids or alkylolamines, such as triamylolamines, or mixtures thereof to disperse solids, such as bentonite and barite, in an oil- or solvent-based drilling fluid containing silicon esters for the purposes of wellbore sealing and stabilisation. U.S. Pat. No. 2,596,844 describes a mixture of the aluminium salt of a fatty acid and the free fatty acid to gel hydrocarbons for their use as fracturing fluids. The addition of water-soluble amines, such as ethanolamine, to the gelled hydrocarbon reduced its viscosity and enabled it to be removed from the fracture. The use of the ethanolamine salts of long-chain carboxylic acids as emulsifiers in water-based drilling fluids has been described in U.S. Pat. Nos. 5,593,954 and 5,707,940. US 2003/0017953 discloses the use of triethanolamine to increase the thermal stability of water-soluble synthetic polymers, such as polyethylene glycols, in wellbore service fluids. WO 2004/018586 describes an oil-soluble condensation product formed between dimer and trimer fatty acids, such as oleic acid dimers and trimers, and diethanolamine for use as a suspending agent in oil-based drilling fluids. The condensation reaction formed an ester when one equivalent of dimer fatty acid and two equivalents of diethanolamine were heated in the temperature range 160-177° C. for 30 to 60 minutes. The condensation reaction products were not ionic and not soluble in water or aqueous solutions.
GB 1100051 describes the use of non-Newtonian solutions of various surfactants in aqueous media for the purposes of extracting oil by pumping the liquid from an injection well to a production well. The aqueous surfactant solutions are described by the general formula(X)(A),[(Y)(B)]a,[(Y)(C)]b where X is alkali metal, ammonium, amine and alkylolamine ions; A is oleate, palmitate, elaidate and stearate; Y is potassium, sodium and ammonium, B is a halide; and C is hydroxide and carbonate. The subscript a refers to a concentration range 0 to 5 weight percent and the subscript b refers to the concentration sufficient to give a pH value greater than 7. The surfactant (X)(A) was generally maintained at a concentration below 1 weight percent when the concentration of the salt (Y)(B) was high (typically>3 weight percent) to ensure the viscosity of the solution was sufficiently low to allow injection into a porous medium. The viscosities of the surfactant solutions were typically 100 cP at shear rates in the range 1-10 s−1 and below about 50 cP at a shear rate of 100 s−1 when the temperature was below about 50° C. Further, the viscosities of the surfactant solutions decreased with increasing temperature, particularly at low shear rates. The solutions all exhibited some shear thickening behaviour in the range of shear rates 0.1-15 s−1. For use at higher temperatures, saturated fatty acids ((H)(A) in the above notation, with H as hydrogen) were considered more suitable.
U.S. Pat. Nos. 6,239,183 and 6,506,710 describe a viscoelastic surfactant solution for use in wellbore service fluids, such as fracturing fluids, based on non-ionic amidoamine oxides with the general formula:
where R1 is an aliphatic hydrophobe (saturated or unsaturated and branched or straight chain) of 7-30 carbon atoms, R5 is hydrogen or an alkyl (or hydroxyalkyl) group and R2, R3 and R4 are various short chain aliphatic groups. Alkanolamines, such as ethanolamine, were added to the viscoelastic surfactant solutions formed by the non-ionic amidoamine oxide for the purposes of corrosion inhibition. The alkanolamines did not react with the surfactant to form any form of salt. Aqueous viscoelastic surfactant solutions were generated using an alkyl sarcosinate surfactant of the general form:
where R1 is an alkyl chain (saturated or unsaturated) having 12-24 carbon atoms, R2 is hydrogen or a short chain alkyl group and X is a carboxylate, sulphate or sulphonate ionic headgroup. The acid headgroup could be neutralized with monovalent cations, such as sodium, potassium or ammonium, or ethanolamines, such as ethanolamine and triethanolamine.Definitions
The term “optionally substituted”, as used herein, pertains to a parent group which may be unsubstituted or which may be substituted with one or more substitutents. The term “substituent” is used herein in the conventional sense and refers to a chemical moiety which is covalently attached to, or if appropriate, fused to, a parent group. A wide variety of substituents are well known, and methods for their formation and introduction into a variety of parent groups are also well known.
The terms “hydrocarbo” and “hydrocarbyl”, when used herein, pertain to compounds and/or groups which have only carbon and hydrogen atoms.
The term “aliphatic”, when used herein, pertains to compounds and/or groups which are linear or branched, but not cyclic.
The term “alkyl”, as used herein, pertains to a monovalent moiety obtained by removing a hydrogen atom from a carbon atom of a hydrocarbon compound which may be saturated or unsaturated (e.g., partially unsaturated, fully unsaturated).
The term “alkylol”, as used herein, pertains to an alkyl group which has one or more hydroxy substituents.
By a “straight chain” we mean a chain of consecutively linked atoms, all of which or the majority of which are carbon atoms. Side chains may branch from the straight chain.
By a “viscoelastic” fluid we mean that the elastic (or storage) modulus G′ of the fluid is equal to or greater than the loss modulus G″ as measured using an oscillatory shear rheometer (such as a Bohlin CVO 50) at a frequency of 1 Hz. The measurement of these moduli is described in An Introduction to Rheology, by H. A. Barnes, J. F. Hutton, and K. Walters, Elsevier, Amsterdam (1997)